Investigation on the Structure and Mechanical Properties of Highly Tunable Elastomeric Silk Fibroin Hydrogels Cross-Linked by γ-Ray Radiation

Silk fibroin (SF) is a natural polymer with low immunogenicity and good biocompatibility. However, most silk-based hydrogels formed through chemical or physical cross-linking are brittle, the preparation of which also inevitably introduces cytotoxic cross-linking agents. Herein, a simple strategy is presented for synthesizing SF hydrogels with tunable mechanical properties by combining gamma-ray radiation with ethanol treatment. Neither toxic initiators nor chemical agents are utilized during the whole preparation procedure. For soft hydrogels, the compressive moduli are less than 29 kPa (SF-S hydrogels), while for tough hydrogels, the compressive moduli are between 1.21 and 2.41 MPa (SF-D hydrogels). Specifically, gamma-ray radiation makes SF form uniform and stable chemical cross-linking sites within and between molecular chains, resulting in soft and highly elastic SF hydrogels. The physical cross-linking via ethanol treatment leads to the self-assembly of fibroin chains, transforming those soft hydrogels to tough hydrogels. These double cross-linked SF hydrogels (SF-D hydrogels) exhibit excellent mechanical strength. Effects of various cross-linking conditions on the secondary structure, pore structure, mechanical properties, gelation degree, swelling, and in vitro degradation properties are explored. A series of cell experiments demonstrate that the SF hydrogels with different mechanical strength can stimulate the expression of specific genes of rat bone marrow mesenchymal stem cells (BMSCs) in various differentiation directions. These results also show the application prospects in tissue engineering by customizing hydrogels for the mechanical strength of different tissues.